It depends.

I think it is more of a guideline than a HR directive that all schools must abide by.

I know of instances where the promoted person still held a C+ or B. These people are usually GEO scale KPs. If you think about it, it isn't fair for a newly promoted GEO4/5 to be given the default C by their school, since they are likely still going to do and contribute more than others of the same grade, by virtue of their appointment.

But if you have no key appointments in school, then your "default" is likely to be C.